elm install breaks with multiple package names

[jjant]

Quick Summary:

Trying to install many packages with one command throws a runtime error on the console.

SSCCE

elm install elm/json elm-community/webgl

• Elm: 0.19
• Operating System: OS X

Additional Details

Exact error is

CallStack (from HasCallStack):
  error, called at ui/terminal/src/Terminal/Args/Error.hs:281:13 in main:Terminal.Args.Error

[evancz]

With 0.19.1 it does not crash, but the error message is still sort of weird:

I was not expecting these arguments:

    elm/json
    elm-community/webgl

Try removing them?

The issue is that it only accepts one package at a time. I'm going to transfer to the error message catalog for further improvement.